Subterranean formations that contain hydrocarbons are sometimes non-homogeneous in their composition along the lengths of wellbores that extend into such formations. It is sometimes desirable to treat and/or otherwise manage the formation and/or the wellbore differently in response to the differing formation composition. Additionally, tools can be provided along the length of the wellbore which may need activation. One way to isolate zones or to activate tools is by use of an obturator. An obturator may be a ball, for example. The obturator can be dropped from the surface and/or pumped through a tubular work string. During passage through the work string, the obturator may strike a tool for activation. Alternatively, the obturator may land on a seat provided in the work string, or alternatively pass through one or more seats. The seats are provided with a bore having a particular diameter which permits or prevents passage of the obturator, depending on the corresponding size of the obturator. When landing on a seat, an obturator may block the bore in the seat and thus seal off the lower portion of the tubular work string.